1. Field of the Invention
The present invention relates to a wide-angle lens having a taking angle of view of about 100 degrees used in a surveillance camera or the like; and, in particular, to a wide-angle lens having a sufficiently long back focus between the lens and its image surface.
2. Description of the Prior Art
In general, for viewing or capturing a wider area, lenses usable at a wide angle of about 100 degrees are demanded in TV cameras for commercial use which are employed as video cameras, surveillance cameras, and the like. For such a demand, there have been proposed wide-angle lenses disclosed in Japanese Unexamined Patent Publication Nos. 9-80303 and 9-127413.
In the wide-angle lenses disclosed in Japanese Unexamined Patent Publication Nos. 9-80303 and 9-127413, however, back focus is less than three times the focal length thereof. For disposing a three-color-separating optical system between the lens and the image surface, a longer back focus is demanded. Specifically, a back focus of about five times the focal length is required as a back focus sufficient for disposing the three-color-separating optical system.
Also, when disposing a three-color-separating optical system between a wide-angle lens and its image surface, the problem of color shading is needed to be taken into account. Luminous fluxes emitted from the wide-angle lens would be incident on a dichroic film of the three-color-separating optical system with their angles greater as they are directed nearer the corner portions of the screen. Color shading is a phenomenon in which, as the angle of light beam incident on the dichroic film varies, the optical path length within the thin film would vary, thus changing its spectral characteristics, whereby colors would appear differently between the upper and lower parts of the screen.
It is demanded for a wide-angle lens coexisting with a three-color-separating lens to suppress the occurrence of such color shading and favorably correct various kinds of aberration, while having a sufficient back focus.